Efficient Mid-Infrared Germanium Variable Optical Attenuator Fabricated by Spin-on-Glass Doping

We demonstrated an efficient carrier-injection germanium (Ge) variable optical attenuator (VOA) operating at 1.95 mu m on a Ge-on-insulator (GeOI) platform for mid-infrared (MIR) integrated photonics. Numerical analysis illustrated that the Ge VOA can realize a significantly higher modulation efficiency than a Si VOA owing to the larger free-carrier absorption in Ge at MIR wavelengths. We proved that by introducing phosphorus-doped spin-on-glass (P-SOG) doping technology into the formation of a Ge lateral PIN junction, we can improve the doping concentration in the n(+) region as compared with P ion implantation, resulting in improved modulation efficiencies of 380-460 dB/A for Ge VOAs. The P-SOG-doped Ge VOA exhibited a comparable modulation efficiency to a Si VOA operating at 1.55 mu m despite the shorter carrier lifetime of 0.4-0.8 ns determined by numerical fitting. Thus, the Ge VOA formed by SOG doping has promising applications in communication and sensing at MIR wavelengths.